The Glenfield 30A is a tip-cap style model. They often shoot just a bit better than barrel band style models (Model 30 [no A] for Glenfield).
It should be just as reliable as any 336 in the same condition.
It'll be plenty good for deer or milk jugs. I would expect 4"+ 100-yd groups with iron sights, and 2-3" groups if scoped. If it shoots better than that, great.
I have a 30A barrel that I cut to 16.125", which hovered right at 1 MoA before being recrowned (and it has not been tested again since). I have another Model 30 barrel that was reamed to .307 Win and shot sub-MoA with Hornady Superformance ammo (160 FTX) until a "gunsmith" ruined it by putting a hardened pin in the muzzle to turn the barrel into a stationary threading die.
While it is generally accepted that Glenfield 30s and 30As are "the same thing" as a Marlin 336, and I do agree that the majority of parts were standard production, I have found quite a few Glenfield parts that are out of spec or have finish problems and are stamped with a G after bluing. (Particularly slightly misshapen receivers, and finger levers and trigger plates with finish or dimensional problems.)
I believe these G-stamped parts were rejects from 336 assembly, but not bad enough to be scrapped, and then set aside for use on Glenfields.
So, if you ever need to replace parts, there is a chance that they may require more fitting than would be typical in a 336.